1. Field of the Invention.
The present invention relates to position sensing generally and, more particularly, but not by way of limitation, to a novel device and method for measuring angular speed and position.
2. Background Art.
There are a number of known devices for measuring angular position and/or speed of a rotating member. These are useful in many applications and, recently, have been employed in antiskid brake systems (ABS) and for vehicular traction control.
Most ABS systems in use today rely upon passive variable reluctance sensing elements disposed in proximity to a toothed iron wheel to detect the speed of rotation of a wheel or drive shaft. Such devices suffer from two primary disadvantages: (1) the output signal of the sensing device degrades unless a close tolerance is kept between the sensor and the toothed wheel (typically 0.015 inches), and (2) the amplitude of the output signal decreases as the rotational speed decreases. The former disadvantage results in high manufacturing cost and susceptibility to minor dislocations between the sensor and the toothed wheel, leading to reliability problems. The second disadvantage renders this type of device of limited utility at low speeds. A complicated arrangement of sensors and wheel teeth must be employed if absolute angular position information is to be derived therefrom.
The foregoing systems typically employ "off-axis" sensors, with the sensors being radially spaced apart from the magnetic source.
Examples of alternative magnetic means of measuring shaft rotation are evident in the literature. Three which exemplify "on-axis" detection means are discussed below. By "on-axis" detection is meant those cases in which the directional magnetic sensor is located on the axis of rotation of the magnet, which is frequently attached to the distal end of the rotating shaft, with the N-S poles radially oriented with respect to the shaft. Such sensors measure the direction of the magnetic field in the equatorial region of the magnetic source (comparable to the field found at the earth's equator.) As the dipolar magnetic source rotates, the field strength on the axis of rotation remains constant and only the direction changes.
Examples of on-axis sensors include the work of W. Willrafen (U.S. Pat. No. 4,646,011, issued Feb. 24, 1987) who determined wind direction by placing a magnetic source on the end of a shaft which was axially rotated by the wind, causing the direction of the field to change at the on-axis location of a solid state compass. Similarly, M. E. Coburn (U.S. Pat. No. 4,647,853, issued Mar. 3, 1987) determined the rotational rate of a mud driven well drill bit by placing a dipolar magnetic source on the top of the rotating drill shaft and monitoring the output from a three axis magnetometer mounted on the axis of rotation of the drill bit, adjacent to the rotating magnet.
A third example of an on-axis sensor is the patent of S. Uemura (U.S. Pat. No. 5,252,919, issued Oct. 12, 1993) who determined shaft rotation over a limited arc using a magnetic source attached to the rotating shaft and two directional magnetic sensor disposed on the axis of rotation at predetermined angles.
Other rotation detection means which utilize magnetic fields include such devices as that of F. Kuhrt (U.S. Pat. No. 3,317,829, issued May 2, 1967) which utilizes the tone generated by the passage of permanent magnets, embedded in a rotating disk, past a magnetic sensor to determine rotational speed. Such a device is only capable of measuring rotational speed, however, not angular position, and is limited in accuracy by the number of permanent magnets embedded in the disk. Its operation is very similar to the prior art which utilizes a tone wheel and a variable reluctance sensor.
Accordingly, it is a principal object of the present invention to provide an angular speed and position measuring device which is economical to install and which does not require critical physical alignment.
It is another object of the invention to provide such a measuring device which can be calibrated after installation.
It is another object of the invention to provide such a measuring device which can accept minor dislocations in the field.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.